Threshold voltage setting with boosting read scheme

ABSTRACT

Methods for reducing read disturb using NAND strings with poly-silicon channels and p-type doped source lines are described. During a boosted read operation for a selected memory cell transistor in a NAND string, a back-gate bias or bit line voltage may be applied to a bit line connected to the NAND string and a source line voltage greater than the bit line voltage may be applied to a source line connected to the NAND string; with these bias conditions, electrons may be injected from the bit line and annihilated in the source line during the read operation. To avoid leakage currents through NAND strings in non-selected memory blocks, the threshold voltages of source-side select gate transistors of the NAND strings may be set to a negative threshold voltage that has an absolute voltage value greater than the source line voltage applied during the read operation.

BACKGROUND

Semiconductor memory is widely used in various electronic devices suchas cellular telephones, digital cameras, personal digital assistants,medical electronics, mobile computing devices, and non-mobile computingdevices. Semiconductor memory may comprise non-volatile memory orvolatile memory. A non-volatile memory allows information to be storedand retained even when the non-volatile memory is not connected to asource of power (e.g., a battery). Examples of non-volatile memoryinclude flash memory (e.g., NAND-type and NOR-type flash memory) andElectrically Erasable Programmable Read-Only Memory (EEPROM).

Both flash memory and EEPROM utilize floating-gate transistors. For eachfloating-gate transistor, a floating gate is positioned above andinsulated from a channel region of the floating-gate transistor. Thechannel region is positioned between source and drain regions of thefloating-gate transistor. A control gate is positioned above andinsulated from the floating gate. The threshold voltage of thefloating-gate transistor may be controlled by setting the amount ofcharge stored on the floating gate. The amount of charge on the floatinggate is typically controlled using Fowler-Nordheim (F-N) tunneling orhot-electron injection. The ability to adjust the threshold voltageallows a floating-gate transistor to act as a non-volatile storageelement or memory cell. In some cases, more than one data bit per memorycell (i.e., a multi-level or multi-state memory cell) may be provided byprogramming and reading multiple threshold voltages or threshold voltageranges.

NAND flash memory structures typically arrange multiple floating-gatetransistors in series with and between two select gates. Thefloating-gate transistors in series and the select gates (e.g., thesource-side select gate and the drain-side select gate) may be referredto as a NAND string. In recent years, NAND flash memory has been scaledin order to reduce cost per bit. However, as process geometries shrink,many design and process challenges are presented. These challengesinclude increased neighboring word line interference, reduced dataretention, and increased leakage currents.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts one embodiment of a NAND string.

FIG. 2 depicts one embodiment of the NAND string of FIG. 1 using acorresponding circuit diagram.

FIG. 3A depicts one embodiment of a memory block including a pluralityof NAND strings.

FIG. 3B depicts one embodiment of possible threshold voltagedistributions for a three-bit-per-cell memory cell.

FIG. 3C depicts one embodiment of a NAND string during a programmingoperation.

FIG. 4A depicts one embodiment of a vertical NAND structure.

FIG. 4B depicts one embodiment of a cross-sectional view taken alongline X-X of FIG. 4A.

FIGS. 4C-4D depict various embodiment of vertical NAND structures.

FIG. 4E depicts one embodiment of a schematic representation of aninverted NAND string.

FIG. 5 depicts one embodiment of a non-volatile storage system.

FIG. 6 depicts one embodiment of a sense block.

FIG. 7A depicts one embodiment of a set of threshold voltagedistributions for a four-state memory device in which each storageelement stores two bits of data.

FIG. 7B depicts one embodiment of a first pass of a two-pass programmingtechnique.

FIG. 7C depicts one embodiment of a second pass of the two-passprogramming technique.

FIG. 7D depicts one embodiment of a first pass of another two-passprogramming technique.

FIG. 7E depicts one embodiment of a second pass of the two-passprogramming technique.

FIG. 7F depicts one embodiment of a series of program and verify pulseswhich are applied to a selected word line during a programmingoperation.

FIG. 8A depicts one embodiment of a vertical NAND string that connectsto a p-type doped source line.

FIG. 8B depicts one embodiment of the vertical NAND string depicted inFIG. 8A during a read operation.

FIG. 8C depicts one embodiment of a conventional NAND string with ann-type source line and an n-type bit line during a read operation.

FIG. 8D depicts one embodiment of a NAND string with a p-type sourceline and an n-type bit line during a read operation.

FIG. 8E depicts one embodiment of the NAND string of FIG. 8D during asecond read operation in which the bit line voltage has been reducedfrom 1.0V to 0.5V.

FIG. 8F depicts one embodiment of an unselected NAND string within anunselected memory block.

FIG. 8G depicts another embodiment of an unselected NAND string withinan unselected memory block.

FIG. 8H depicts another embodiment of an unselected NAND string withinan unselected memory block.

FIGS. 9A-9B depict a flowchart describing one embodiment of a processfor performing read operations.

FIG. 9C depicts a flowchart describing another embodiment of a processfor performing a read operation.

FIG. 9D depicts a flowchart describing an alternative embodiment of aprocess for performing a read operation.

DETAILED DESCRIPTION

Technology is described for reducing read disturb and reducing the costof manufacturing non-volatile memory using NAND strings (e.g., verticalor horizontal NAND strings) with silicon-based or poly-silicon channelsand p-type doped source lines. During a boosted read operation for aselected memory cell transistor in a NAND string that utilizes a p-typedoped source line, a back-gate bias or bit line voltage may be appliedto a bit line connected to the NAND string and a source line voltagegreater than the bit line voltage may be applied to a source lineconnected to the NAND string; with these bias conditions, electrons maybe injected from the bit line and annihilated in the source line duringthe read operation. During the boosted read operation, the bit line mayact as the source of free electrons with electron-hole recombinationoccurring at the source line and current flowing from the source lineinto the bit line; in contrast, during a conventional read operationwith a NAND string that utilizes an n-type doped source line, the sourceline acts as the source of free electrons with current flowing from thebit line towards the source line.

The application of the back-gate bias to the bit line during the readoperation of the NAND string that utilizes a p-type doped source linemay improve the performance of the read operation due to the suppressionof short channel effects and the reduction in neighboring word lineinterference. Neighboring word line interference or the amount ofshifting in the programmed threshold voltage of a memory cell transistordue to a neighboring memory cell transistor connected to a neighboringword line being programmed subsequent to the prior programming of thethreshold voltage of the memory cell transistor is greatest when theneighboring memory cell transistor is programmed to the highest datastate (e.g., to the G-state in a three-bit per cell memory). As a NANDstring with a polycrystalline silicon (or poly-silicon) channel that isconnected to a p-type doped (e.g., a boron-doped) source line may allowfor higher programmed threshold voltages for the same appliedprogramming voltage (e.g., Vpgm) to the selected word line as that usedin a conventional NAND string with an n-type doped source line, theamount of neighboring word line interference during the programming ofneighboring memory cell transistors may be reduced. Moreover, theapplication of the back-gate bias to the bit line during the readoperation may also facilitate current sensing for sense amplifiersconnected to the bit line.

In some embodiments, a memory block may include a plurality of NANDstrings or NAND flash memory structures, such as vertical NANDstructures or bit cost scalable (BiCS) NAND structures. Each NANDstructure may comprise a NAND string that utilizes a p-type doped sourceline. A controller (or one or more control circuits) in communicationwith a memory block may determine a source line voltage to be applied toa source line connected to a NAND string within the memory block priorto a boosted read operation, determine a threshold voltage level for athreshold voltage of a source-side select gate transistor for the NANDstring based on the source line voltage to be applied to the source lineduring the boosted read operation, and set the threshold voltage of thesource-side select gate transistor for the NAND string to the thresholdvoltage level prior to performing the boosted read operation. In oneexample, prior to the boosted read operation, the threshold voltage ofthe source-side select gate transistor may be set to a negativethreshold voltage (e.g., to minus three volts or −3V).

In one embodiment, the threshold voltage of the source-side select gatetransistor may be set during testing or sorting of memory die (e.g.,during wafer soft or die sort) and remain fixed during operating of thememory die. In other embodiments, the threshold voltage of thesource-side select gate transistor may be initially set during testingor sorting of memory die and then be dynamically adjusted over time on aper memory die basis or a per page basis based on chip temperatureand/or the number of program/erase cycles. In one example, if the chiptemperature is greater than a threshold temperature, then the thresholdvoltage of the source-side select gate transistor may be reduced or mademore negative (e.g., adjusted from −2V to −3V). In another example, ifthe number of program/erase cycles for a particular page has exceeded athreshold number of cycles (e.g., is more than 15 cycles), then thethreshold voltage of the source-side select gate transistor may bereduced or made more negative (e.g., adjusted from −1V to −2V). Thethreshold voltage of the source-side select gate transistor may beperiodically updated (e.g., every 10 ms) based on the chip temperatureand/or the number of program/erase cycles.

To set the threshold voltage of the source-side select gate transistoror a programmable transistor within a NAND string to a negativethreshold voltage, an erase operation may be performed. During the eraseoperation, all programmable transistors within a memory block may beerased or have their threshold voltages set to a negative thresholdvoltage. In some cases, to set the threshold voltage of a source-sideselect gate transistor to a more negative threshold voltage than otherprogrammable transistors, the voltage applied to the source-side selectgate line connected to the control gate of the source-side select gatetransistor may be reduced during the erase operation. For example,during the erase operation, the source-side select gate line may be setto 0V, while word lines connected to control gates of memory celltransistors are set to 1V.

In some cases, a controller may determine whether to program or erasethe threshold voltage of the source-side select gate transistor for theNAND string to the determined threshold voltage level or both thethreshold voltage of the source-side select gate transistor and thethreshold voltage of a dummy word line transistor on the source-side ofthe NAND string that is arranged between the source-side select gatetransistor and the memory cell transistors of the NAND string based onthe source line voltage to be applied to the source line during theboosted read operation or based on a voltage difference between thesource line voltage and the bit line voltage to be applied during theboosted read operation. In one example, if the voltage differencebetween the source line voltage and the bit line voltage to be appliedduring the boosted read operation is greater than a threshold voltagedifference (e.g., is greater than 2V), then the threshold voltages forboth the source-side select gate transistor and the dummy word linetransistor may be erased to a negative threshold voltage (e.g., to −2V);however, if the voltage difference between the source line voltage andthe bit line voltage to be applied during the boosted read operation isnot greater than the threshold voltage difference (e.g., is less than2V), then the threshold voltage for the source-side select gatetransistor may be erased to a negative threshold voltage (e.g., to −3V)and the threshold voltage for the source-side dummy word line transistormay be programmed with a positive threshold voltage (e.g., to +3V).

In another example, if the source line voltage to be applied during theboosted read operation is greater than a threshold source line voltage(e.g., is greater than 2V), then the threshold voltage for thesource-side select gate transistor may be set to a negative thresholdvoltage with an absolute value that is greater than the threshold sourceline voltage (e.g., if the threshold source line voltage is 2V, then thenegative threshold voltage for the source-side select gate transistormay be −2.2V); however, if the source line voltage to be applied duringthe boosted read operation is not greater than the threshold source linevoltage (e.g., is less than 0.7V), then the threshold voltage for thesource-side select gate transistor may be programmed with a non-negativethreshold voltage (e.g., if the threshold source line voltage to beapplied during the read operation is 0.5V, then the threshold voltagefor the source-side select gate transistor may be set to 0V or 3V).

One technical issue with using NAND strings that utilize p-type dopedsource lines and biasing the source lines to a source line voltagegreater than the bit line voltages applied to the bit lines connected tothe NAND strings during a read operation is that substantial leakagecurrent may occur within unselected NAND strings within unselectedmemory blocks that are connected to the same source lines as theselected memory blocks. As both source lines and bit lines may extendacross both selected memory blocks and unselected memory blocks,unwanted channel current through the unselected NAND strings due to theforward bias applied to the source lines may cause increased powerconsumption and reduced battery lifetime. To avoid the unwanted leakagecurrents through the NAND strings in non-selected memory blocks, thethreshold voltage levels of the source-side select gates may be set to anegative threshold voltage (e.g., to a negative threshold voltage levelthat has an absolute voltage value that is greater than the positivesource line voltage applied to the source lines during the readoperation). One technical benefit of setting the threshold voltages ofthe source-side select gates to a negative threshold voltage prior tothe read operation is that holes injected into the channels of theunselected NAND strings from the source lines during the read operationmay be blocked and the leakage currents through the unselected NANDstrings may be significantly reduced.

In one embodiment, a non-volatile storage system may include one or moretwo-dimensional arrays of non-volatile memory cells. The memory cellswithin a two-dimensional memory array may form a single layer of memorycells and may be selected via control lines (e.g., word lines and bitlines) in the X and Y directions. In another embodiment, a non-volatilestorage system may include one or more monolithic three-dimensionalmemory arrays in which two or more layers of memory cells may be formedabove a single substrate without any intervening substrates. In somecases, a three-dimensional memory array may include one or more verticalcolumns of memory cells located above and orthogonal to a substrate. Inone example, a non-volatile storage system may include a memory arraywith vertical bit lines or bit lines that are arranged orthogonal to asemiconductor substrate. The substrate may comprise a silicon substrate.

In some embodiments, a non-volatile storage system may include anon-volatile memory that is monolithically formed in one or morephysical levels of arrays of memory cells having an active area disposedabove a silicon substrate. The non-volatile storage system may alsoinclude circuitry associated with the operation of the memory cells(e.g., decoders, state machines, page registers, or control circuitryfor controlling the reading or programming of the memory cells). Thecircuitry associated with the operation of the memory cells may belocated above the substrate or located within the substrate.

In some embodiments, a non-volatile storage system may include amonolithic three-dimensional memory array. The monolithicthree-dimensional memory array may include one or more levels of memorycells. Each memory cell within a first level of the one or more levelsof memory cells may include an active area that is located above asubstrate (e.g., above a single-crystal substrate or a crystallinesilicon substrate). In one example, the active area may include asemiconductor junction (e.g., a P-N junction). The active area mayinclude a portion of a source or drain region of a transistor. Inanother example, the active area may include a channel region of atransistor.

FIG. 1 depicts one embodiment of a NAND string 90. FIG. 2 depicts oneembodiment of the NAND string of FIG. 1 using a corresponding circuitdiagram. As depicted, NAND string 90 includes four transistors, 100,102, 104, and 106, in series between a first select gate 120 (i.e., adrain-side select gate) and a second select gate 122 (i.e., asource-side select gate). Select gate 120 connects the NAND string 90 toa bit line 126. Select gate 122 connects the NAND string 90 to a sourceline 128. Select gate 120 is controlled by applying the appropriatevoltage to control gate 120CG (i.e., via select line SGD of FIG. 2).Select gate 122 is controlled by applying the appropriate voltage tocontrol gate 122CG (i.e., via select line SGS of FIG. 2). Each of thetransistors 100, 102, 104, and 106 includes a control gate and afloating gate. For example, transistor 100 includes control gate 100CGand floating gate 100FG, transistor 102 includes control gate 102CG andfloating gate 102FG, transistor 104 includes control gate 104CG andfloating gate 104FG, and transistor 106 includes control gate 106CG andfloating gate 106FG. Control gates 100CG, 102CG, 104CG, and 106CG areconnected to word lines WL3, WL2, WL1, and WL0, respectively.

Note that although FIGS. 1 and 2 show four floating-gate transistors inthe NAND string, the use of four floating-gate transistors is onlyprovided as an example. A NAND string may have less than or more thanfour floating-gate transistors (or memory cells). For example, some NANDstrings may include 16 memory cells, 32 memory cells, 64 memory cells,128 memory cells, etc. The discussion herein is not limited to anyparticular number of memory cells in a NAND string. One embodiment usesNAND strings with 66 memory cells, where 64 memory cells are used tostore data and two of the memory cells may be referred to as dummymemory cells because they do not store user accessible data.

A typical architecture for a flash memory system using a NAND flashmemory structure includes a plurality of NAND strings within a memoryblock. A memory block may comprise a unit of erase. In some cases, theNAND strings within a memory block may share a common well (e.g., aP-well). Each NAND string may be connected to a common source line byits source-side select gate (e.g., controlled by select line SGS) andconnected to its associated bit line by its drain-side select gate(e.g., controlled by select line SGD). Typically, each bit line runs ontop of (or over) its associated NAND string in a direction perpendicularto the word lines and is connected to a sense amplifier.

In some embodiments, during a programming operation, storage elementsthat are not to be programmed (e.g., storage elements that havepreviously completed programming to a target data state) may beinhibited or locked out from programming by boosting associated channelregions (e.g., self-boosting the channel regions via word linecoupling). An unselected storage element (or unselected NAND string) maybe referred to as an inhibited or locked out storage element (orinhibited NAND string) as it is inhibited or locked out from programmingduring a given programming iteration of a programming operation.

Although technology using NAND-type flash memory is described herein,the technology disclosed herein may also be applied to other types ofnon-volatile storage devices and architectures (e.g., NOR-type flashmemory). Moreover, although technology using floating-gate transistorsis described herein, the technology described herein may also be appliedto or used with other memory technologies including those that employcharge trapping, phase-change (e.g., chalcogenide materials), orstate-change materials.

FIG. 3A depicts one embodiment of a memory block including a pluralityof NAND strings. As depicted, each NAND string includes (Y+1) memorycells. Each NAND string is connected to one bit line out of (X+1) bitlines on the drain side (i.e., one bit line of bit lines BL0-BLX) via adrain-side select gate controlled by the drain-side selection signalSGD. Each NAND string is connected to a source line (source) via asource-side select gate controlled by source-side selection signal SGS.In one embodiment, the source-side select gate controlled by source-sideselection signal SGS and the drain-side select gate controlled by thedrain-side selection signal SGD may comprise transistors withoutfloating gates or transistors that include a floating gate structure.

In some embodiments, in order to save space on a semiconductor die, twoadjacent NAND strings (or other grouping in memory cells) may share acommon bit line (i.e., a shared-bit-line memory architecture). In somecases, more than two NAND strings may share a common bit line. In oneexample, the signal SGD may be replaced by two drain-side selectionsignals SGD1 and SGD2. Each NAND string of the pair would then have twodrain-side select gates, each connected to a different drain-sideselection signal of the two drain side selection signals SGD1 and SGD2.One of the two drain-side select gates for each NAND string may be adepletion mode transistor with its threshold voltage lower than 0 volts.One potential problem with using two select gates on the drain side ofeach NAND string is that two drain-side select gates (as compared to onedrain-side select transistor) requires more area on the die. Therefore,from an integrated circuit area standpoint, it may be beneficial to onlyuse one drain-side selection gate for each NAND string and then connecteach NAND string of the pair with only one of the two drain-sideselection signals.

In one embodiment, during a programming operation, when programming amemory cell, such as a NAND flash memory cell, a program voltage may beapplied to the control gate of the memory cell and the corresponding bitline may be grounded. These programming bias conditions may causeelectrons to be injected into the floating gate via field-assistedelectron tunneling, thereby raising the threshold voltage of the memorycell. The program voltage applied to the control gate during a programoperation may be applied as a series of pulses. In some cases, themagnitude of the programming pulses may be increased with eachsuccessive pulse by a predetermined step size. Between programmingpulses, one or more verify operations may be performed. During theprogramming operation, memory cells that have reached their intendedprogramming states may be locked out and inhibited from programming byboosting the channel regions of the program inhibited memory cells.

In one embodiment, memory cells may be erased by raising the p-well toan erase voltage (e.g., 20 volts) for a sufficient period of time andgrounding the word lines of a selected block of memory cells while thesource and bit lines are floating. These erase bias conditions may causeelectrons to be transferred from the floating gate through the tunnelingoxide, thereby lowering the threshold voltage of the memory cells withinthe selected block. In some cases, an erase operation may be performedon an entire memory plane, on individual blocks within a memory plane,or another unit of memory cells.

In some embodiments, during verify operations and/or read operations, aselected word line may be connected (or biased) to a voltage, a level ofwhich is specified for each read and verify operation in order todetermine whether a threshold voltage of a particular memory cell hasreached such level. After applying the word line voltage, the conductioncurrent of the memory cell may be measured (or sensed) to determinewhether the memory cell conducted a sufficient amount of current inresponse to the voltage applied to the word line. If the conductioncurrent is measured to be greater than a certain value, then it isassumed that the memory cell turned on and the voltage applied to theword line is greater than the threshold voltage of the memory cell. Ifthe conduction current is not measured to be greater than the certainvalue, then it is assumed that the memory cell did not turn on and thevoltage applied to the word line is not greater than the thresholdvoltage of the memory cell.

There are many ways to measure the conduction current of a memory cellduring a read or verify operation. In one example, the conductioncurrent of a memory cell may be measured by the rate it discharges orcharges a dedicated capacitor in a sense amplifier. In another example,the conduction current of the selected memory cell allows (or fails toallow) the NAND string that included the memory cell to discharge avoltage on the corresponding bit line. The voltage of the bit line (orthe voltage across a dedicated capacitor in a sense amplifier) may bemeasured after a period of time to determine whether the bit line hasbeen discharged by a particular amount or not.

FIG. 3B depicts one embodiment of possible threshold voltagedistributions (or data states) for a three-bit-per-cell memory cell(i.e., the memory cell may store three bits of data). Other embodiments,however, may use more than or less than three bits of data per memorycell (e.g., such as four or more bits of data per memory cell). At theend of a successful programming process (with verification), thethreshold voltages of memory cells within a memory page or memory blockshould be within one or more threshold voltage distributions forprogrammed memory cells or within a distribution of threshold voltagesfor erased memory cells, as appropriate.

As depicted, each memory cell may store three bits of data; therefore,there are eight valid data states S0-S7. In one embodiment, data stateS0 is below 0 volts and data states S1-S7 are above 0 volts. In otherembodiments, all eight data states are above 0 volts, or otherarrangements can be implemented. In one embodiment, the thresholdvoltage distribution S0 is wider than distributions S1-S7.

Each data state S0-S7 corresponds to a unique value for the three bitsstored in the memory cell. In one embodiment, S0=111, S1=110, S2=101,S3=100, S4=011, S5=010, S6=001 and S7=000. Other mappings of data tostates S0-S7 can also be used. In one embodiment, all of the bits ofdata stored in a memory cell are stored in the same logical page. Inother embodiments, each bit of data stored in a memory cell correspondsto different pages. Thus, a memory cell storing three bits of data wouldinclude data in a first page, a second page, and a third page. In someembodiments, all of the memory cells connected to the same word linewould store data in the same three pages of data. In some embodiments,the memory cells connected to a word line can be grouped into differentsets of pages (e.g., by odd and even bit lines).

In some example implementations, the memory cells will be erased tostate S0. From state S0, the memory cells can be programmed to any ofstates S1-S7. Programming may be performed by applying a set of pulseswith rising magnitudes to the control gates of the memory cells. Betweenpulses, a set of verify operations may be performed to determine whetherthe memory cells being programmed have reached their target thresholdvoltage (e.g., using verify levels Vv1, Vv2, Vv3, Vv4, Vv5, Vv6, andVv7). Memory cells being programmed to state S1 will be tested to see iftheir threshold voltage has reached Vv1. Memory cells being programmedto state S2 will be tested to see if their threshold voltage has reachedVv2. Memory cells being programmed to state S3 will be tested to see iftheir threshold voltage has reached Vv3. Memory cells being programmedto state S4 will be tested to see if their threshold voltage has reachedVv4. Memory cells being programmed to state S5 will be tested to see iftheir threshold voltage has reached Vv5. Memory cells being programmedto state S6 will be tested to see if their threshold voltage has reachedVv6. Memory cells being programmed to state S7 will be tested to see iftheir threshold voltage has reached Vv7.

When reading memory cells that store three bits of data, multiple readswill be performed at read compare points Vr1, Vr2, Vr3, Vr4, Vr5, Vr6,and Vr7 to determine which state the memory cells are in. If a memorycell turns on in response to Vr1, then it is in state S0. If a memorycell turns on in response to Vr2 but does not turn on in response toVr1, then it is in state S1. If a memory cell turns on in response toVr3 but does not turn on in response to Vr2, then it is in state S2. Ifa memory cell turns on in response to Vr4 but does not turn on inresponse to Vr3, then it is in state S3. If a memory cell turns on inresponse to Vr5 but does not turn on in response to Vr4, then it is instate S4. If a memory cell turns on in response to Vr6 but does not turnon in response to Vr5, then it is in state S5. If a memory cell turns onin response to Vr7 but does not turn on in response to Vr6, then it isin state S6. If a memory cell does not turn on in response to Vr7, thenit is in state S7.

FIG. 3C depicts one embodiment of a NAND string 300 during a programmingoperation. When programming a storage element (e.g., the storage element316 associated with WL5) of the NAND string 300, a programming voltagemay be applied to the selected word line associated with the storageelement and a low voltage (e.g., ground) may be applied to the bit lineassociated with the storage element. As depicted, the NAND string 300includes a source-side select gate 306, a drain-side select gate 308,and eight word lines WL0-WL7 formed above a substrate 310. V_(SGS) maybe applied to the source-side select gate 306 and V_(SGD) may be appliedto the drain-side select gate 308. The bit line 302 may be biased to VBAand the source line 304 may be biased to V_(SOURCE). During aprogramming operation, a programming voltage, V_(PGM), may be applied toselected word line WL5, which is associated with a selected storageelement 316.

In one example of a boosting mode, when storage element 316 is theselected storage element, a relatively low voltage, V_(LOW) (e.g., 2-6V)may be applied to a source-side word line (WL3), while an isolationvoltage, V_(IS0) (e.g., 0-4V) may be applied to another source-side wordline (WL2), referred to as an isolation word line and a pass voltage,V_(PASS), may be applied to the remaining word lines associated withNAND string 300 (in this case word lines WL0, WL1, WL4, WL6, and WL7).While the absolute values of V_(IS0) and V_(LOW) may vary over arelatively large and partly overlapping range, V_(IS0) may be less thanV_(LOW). In some cases, V_(IS0) may be less than V_(LOW) which is lessthan V_(PASS) which is less than V_(PGM).

FIG. 4A depicts one embodiment of a vertical NAND structure. Thevertical NAND structure includes an inverted NAND string formed abovethe substrate 424 and oriented such that the inverted NAND string isorthogonal to the substrate 424. An inverted NAND string may comprise aNAND string that includes an inverted floating gate transistor with atunneling oxide between a floating gate of the inverted floating gatetransistor and a control gate of the inverted floating gate transistor.The arrangement of the tunneling oxide between the floating gate and thecontrol gate allows the mechanism (e.g., F-N tunneling as the transportmechanism) for programming and/or erase of the inverted floating gatetransistor to occur between the floating gate and the control gaterather than between the floating gate and the channel of the invertedfloating gate transistor. The inverted NAND string may be arrangedwithin a vertical memory hole that is etched through alternating layersof control gate material (e.g., tungsten, nitride, or polysilicon) andinter-gate insulator material (e.g., oxide or silicon dioxide). Asdepicted, the layers of control gate material include layer 417 andlayers 414-416 and the layers of inter-gate insulator material includelayers 418-420. The inter-gate insulator material layer 420 may bearranged above a source line layer 422 (e.g., doped polysilicon) thatmay be arranged above a substrate 424 (e.g., a silicon substrate). Insome cases, a first word line (WL1) may correspond with control gatelayer 414, a second word line (WL0) may correspond with control gatelayer 415, and a source-side select gate line (SGS) may correspond withcontrol gate layer 416.

In one embodiment, within the memory hole a tunneling layer material 408(e.g., including a thin oxide), a floating gate material 410 (e.g.,polysilicon), a dielectric layer 412 (e.g., oxide), and a channel layermaterial 406 (e.g., undoped polysilicon) may be deposited within thememory hole and arranged in order to form the inverted NAND string. Asdepicted in FIG. 4A, the tunneling layer material 408 is arranged withinor inside of the memory hole. The tunneling layer material 408 maycomprise a portion of a multi-layer dielectric stack such as an ONOdielectric stack, which includes alternating layers of silicon dioxide(“O”) and silicon nitride (“N”). In some cases, the tunneling layermaterial 408 may comprise a high-K dielectric material (e.g.,hafnium-based high-K dielectrics or hafnium oxide) that has a dielectricconstant that is greater than that of silicon dioxide. In some cases, acore material layer 404 (e.g., oxide) may be formed within the memoryhole. In other cases, the core material layer 404 may be omitted. A bitline contact layer 402 may be formed at the top of the memory hole andconnect to or directly abut the channel layer material 406. The channellayer material 406 may connect to the source line layer 422 at thebottom of the memory hole. Thus, in this case, the bit line contactlayer 402 connects to the inverted NAND string at the top of the memoryhole and the source line contact layer 422 connects to the inverted NANDstring at the bottom of the memory hole.

In one embodiment, the bit line contact layer 402 may comprise amaterial of a first conductivity type (e.g., n-type) and the source linecontact layer 422 may comprise a material of a second conductivity typedifferent from the first conductivity type (e.g., p-type). In oneexample, the bit line contact layer 402 may comprise an n-type material(e.g., n-type polysilicon) and the source line contact layer 422 maycomprise a p-type material (e.g., p-type polysilicon). In anotherexample, the bit line contact layer 402 may comprise a p-type materialand the source line contact layer 422 may comprise an n-type material(e.g., n-type polysilicon). Thus, in some cases, the inverted NANDstring may include an asymmetric source and drain that may be used toprovide both an electron supply (via the n-type material) and a holesupply (via the p-type material) for memory operations (e.g., program,erase, and read operations) performed using the inverted NAND string.The memory operations may comprise n-channel operations and/or p-channeloperations depending on the bias conditions applied to the inverted NANDstring.

In one embodiment, an inverted NAND string may be formed using a corematerial layer (e.g., an oxide layer or other dielectric layer) that isarranged adjacent to a channel layer (e.g., an undoped polysiliconchannel layer) that is arranged adjacent to a blocking layer (e.g., anoxide layer or other dielectric layer) that is arranged adjacent to afloating gate layer (or a charge trap layer) that is arranged adjacentto a tunneling layer (e.g., a thin oxide) that is arranged adjacent to acontrol gate layer (e.g., tungsten). The tunneling layer may have athickness that is less than the thickness of the blocking layer.

FIG. 4B depicts one embodiment of a cross-sectional view taken alongline X-X of FIG. 4A. As depicted, the inverted NAND string includes aninner core material layer 404 that is surrounded by the channel layermaterial 406 that is surrounded by the dielectric layer 412 that issurrounded by the floating gate material 410 that is surrounded by thetunneling layer material 408 that is surrounded by the control gatematerial layer 417. In one embodiment, FIG. 4A may depict across-sectional view taken along line Y-Y of FIG. 4B. In one embodiment,the inverted NAND string may be formed using a vertical cylindricalstructure or a vertical tapered cylindrical structure. In this case, thedielectric material 412, floating gate material 410, tunneling layermaterial 408, and channel layer material 406 of the inverted NAND stringmay comprise vertical annular structures surrounding the core materiallayer 404. In another embodiment, the inverted NAND string may be formedusing a vertical pillar structure or a vertical rectangular prismstructure.

FIG. 4C depicts another embodiment of a vertical NAND structure. Thevertical NAND structure includes an inverted NAND string formed abovethe substrate 450 and oriented such that the inverted NAND string isorthogonal to the substrate 450. The inverted NAND string may bearranged within a vertical memory hole that is etched throughalternating layers of control gate material (e.g., tungsten, nitride, orpolysilicon) and inter-gate insulator material (e.g., oxide or siliconoxide). As depicted, the layers of control gate material include layers444-445 and the layers of inter-gate insulator material include layers446-447. The inter-gate insulator material layer 447 may be arrangedabove a source line layer 448 (e.g., doped polysilicon) that may bearranged above the substrate 450 (e.g., a silicon substrate). In somecases, a first word line may correspond with control gate layer 444 anda second word line may correspond with control gate layer 445.

In one embodiment, within the memory hole a tunneling layer material 438(e.g., including a thin oxide), a charge trap layer material 440 (e.g.,silicon nitride), a dielectric layer 442 (e.g., oxide), and a channellayer material 436 (e.g., undoped polysilicon) may be deposited withinthe memory hole and arranged in order to form the inverted NAND string.As depicted in FIG. 4C, the tunneling layer material 438 may be arrangedwithin or inside of the memory hole. The tunneling layer material 438may comprise a portion of a multi-layer dielectric stack such as an ONOdielectric stack, which includes alternating layers of silicon dioxide(“O”) and silicon nitride (“N”). In some cases, a core material layer434 (e.g., oxide) may be formed within the memory hole. In other cases,the core material layer 434 may be omitted. A bit line contact layer 432may be formed at the top of the memory hole and connect to the channellayer material 436. The channel layer material 436 may connect to thesource line layer 448 at the bottom of the memory hole. Thus, in thiscase, the bit line contact layer 432 connects to the inverted NANDstring at the top of the memory hole and the source line contact layer448 connects to the inverted NAND string at the bottom of the memoryhole.

In one embodiment, the bit line contact layer 432 may comprise amaterial of a first conductivity type (e.g., n-type) and the source linecontact layer 448 may comprise a material of a second conductivity typedifferent from the first conductivity type (e.g., p-type). In oneexample, the bit line contact layer 432 may comprise an n-type material(e.g., n-type polysilicon) and the source line contact layer 448 maycomprise a p-type material (e.g., p-type polysilicon). In anotherexample, the bit line contact layer 432 may comprise a p-type material(e.g., p-type polysilicon) and the source line contact layer 448 maycomprise an n-type material (e.g., n-type polysilicon). Thus, in somecases, the inverted NAND string may include an asymmetric source anddrain that may be used to provide both an electron supply (via then-type material) and a hole supply (via the p-type material) for memoryoperations (e.g., program, erase, and read operations) performed usingthe inverted NAND string. The memory operations may comprise n-channeloperations and/or p-channel operations depending on the bias conditionsapplied to the inverted NAND string.

FIG. 4D depicts another embodiment of a vertical NAND structure. Thevertical NAND structure includes an inverted NAND string formed abovethe substrate 480 and oriented such that the inverted NAND string isorthogonal to the substrate 480. The inverted NAND string may bearranged within a vertical memory hole that is etched throughalternating layers of control gate material (e.g., tungsten, nitride, orpolysilicon) and inter-gate insulator material (e.g., oxide or siliconoxide). As depicted, the layers of control gate material include layers474-475 and the layers of inter-gate insulator material include layers476-477. The inter-gate insulator material layer 477 may be arrangedabove a source line layer 478 (e.g., doped polysilicon) that may bearranged above the substrate 480 (e.g., a silicon substrate). In somecases, a first word line may correspond with control gate layer 474 anda second word line may correspond with control gate layer 475.

In one embodiment, within the memory hole a tunneling layer material 468(e.g., including a thin oxide), a floating gate material 470 (e.g.,polysilicon), a dielectric layer 472 (e.g., oxide), and a channel layermaterial 466 (e.g., undoped polysilicon) may be arranged in order toform the inverted NAND string. As depicted in FIG. 4D, the tunnelinglayer material 468 is arranged outside of the memory hole structure. Thetunneling layer material 468 may comprise a portion of a multi-layerdielectric stack such as an ONO dielectric stack, which includesalternating layers of silicon dioxide (“O”) and silicon nitride (“N”).In some cases, a core material layer 464 (e.g., oxide) may be formedwithin the memory hole. In other cases, the core material layer 464 maybe omitted. A bit line contact layer 462 may be formed at the top of thememory hole and connect to the channel layer material 466. The channellayer material 466 may connect to the source line layer 478 at thebottom of the memory hole. Thus, in this case, the bit line contactlayer 462 connects to the inverted NAND string at the top of the memoryhole and the source line contact layer 478 connects to the inverted NANDstring at the bottom of the memory hole.

In one embodiment, the bit line contact layer 462 may comprise amaterial of a first conductivity type (e.g., n-type) and the source linecontact layer 478 may comprise a material of a second conductivity typedifferent from the first conductivity type (e.g., p-type). In oneexample, the bit line contact layer 462 may comprise an n-type material(e.g., n-type polysilicon) and the source line contact layer 478 maycomprise a p-type material (e.g., p-type polysilicon). In anotherexample, the bit line contact layer 462 may comprise a p-type materialand the source line contact layer 478 may comprise an n-type material(e.g., n-type polysilicon). Thus, in some cases, the inverted NANDstring may include an asymmetric source and drain that may be used toprovide both an electron supply (via the n-type material) and a holesupply (via the p-type material) for memory operations (e.g., program,erase, and read operations) performed using the inverted NAND string.The memory operations may comprise n-channel operations and/or p-channeloperations depending on the bias conditions applied to the inverted NANDstring.

FIG. 4E depicts one embodiment of a schematic representation of aninverted NAND string. As depicted, the inverted NAND string includeseight transistors in series between a source line (SL) and a bit line(BL). In some embodiments, the eight transistors may correspond with theeight transistors depicted in FIG. 4A, the eight transistors depicted inFIG. 4C, or the eight transistors depicted in FIG. 4D. The gate of thedrain-side select transistor is connected to SGD and the gate of thesource-side select transistor is connected to SGS. Between thedrain-side select transistor and the source-side select transistors issix memory cell transistors connected in series and each connected toone of the word lines WL0 through WL5. Although the inverted NAND stringdepicted in FIG. 4E includes only six memory cell transistors, otherinverted NAND strings may comprise two or more memory cell transistorsin series (e.g., 32 memory cell transistors or 64 memory celltransistors). In one embodiment, a memory cell transistor within aninverted NAND string may comprise an inverted floating gate transistorwith a tunneling layer (e.g., a tunneling oxide) between a floating gateof the inverted floating gate transistor and a control gate of theinverted floating gate transistor. The floating gate may comprisepolysilicon. In another embodiment, a memory cell transistor within aninverted NAND string may comprise an inverted charge trap transistorwith a tunneling layer (e.g., a tunneling oxide) between a charge traplayer of the inverted charge trap transistor and a control gate of theinverted charge trap transistor. The charge trap layer may comprisesilicon nitride.

In some cases, a vertical NAND structure may comprise a vertical NANDstring or a vertical inverted NAND string. A NAND string may comprise astring of floating gate transistors. An inverted NAND string maycomprise a string of inverted floating gate transistors.

FIG. 5 depicts one embodiment of a non-volatile storage system 596including read/write circuits for reading and programming a page (orother unit) of memory cells (e.g., NAND multi-level cells) in parallel.As depicted, non-volatile storage system 596 includes a memory die 598and controller 550. Memory die 598 includes a memory array 501 (e.g., aNAND flash memory array), control circuitry 510, row decoder 530, columndecoder 560, and read/write circuits 565. In one embodiment, access tothe memory array 501 by the various peripheral circuits (e.g., rowdecoders or column decoders) is implemented in a symmetric fashion, onopposite sides of the array, so that the densities of access lines andcircuitry on each side are reduced by half. The memory array 501 isaddressable by word lines via a row decoder 530 and by bit lines via acolumn decoder 560. Word lines and bit lines are examples of memoryarray control lines. The read/write circuits 565 include multiple senseblocks 500 that allow a page of storage elements to be read orprogrammed in parallel. In some cases, controller 550 may be integratedon the memory die 598. Commands and data are transferred between thehost and controller 550 via lines 520 and between the controller 550 andthe memory die 598 via lines 518.

The control circuitry 510 cooperates with the read/write circuits 565 toperform memory operations on the memory array 501. The control circuitry510 includes a state machine 512, an on-chip address decoder 514, and apower control module 516. The state machine 512 provides chip-levelcontrol of memory operations. The on-chip address decoder 514 providesan address interface between that used by the host or a memorycontroller to the hardware address used by the decoders 530 and 560. Thepower control module 516 controls the power and voltages supplied to theword lines and bit lines during memory operations. In one embodiment, apower control module 516 includes one or more charge pumps that maygenerate voltages greater than the supply voltage.

In some embodiments, one or more of the components (alone or incombination), other than memory array 501, may be referred to as amanaging or control circuit. For example, one or more managing orcontrol circuits may include any one of or a combination of controlcircuitry 510, state machine 512, decoders 530/560, power control 516,sense blocks 500, read/write circuits 565, controller 550, and so forth.The one or more managing circuits or the one or more control circuitsmay perform or facilitate one or more memory array operations includingerasing, programming, or reading operations.

In one embodiment, memory array 501 may be divided into a large numberof blocks (e.g., blocks 0-1023, or another amount) of memory cells. Asis common for flash memory systems, the block may be the unit of erase.That is, each block may contain the minimum number of memory cells thatare erased together. Other units of erase can also be used. A blockcontains a set of NAND strings which are accessed via bit lines and wordlines. Typically, all of the NAND strings in a block share a common setof word lines.

Each block may be divided into a particular number of pages. In oneembodiment, a page may be the unit of programming. Other units ofprogramming can also be used. One or more pages of data are typicallystored in one row of memory cells. For example, one or more pages ofdata may be stored in memory cells connected to a common word line. Inone embodiment, the set of memory cells that are connected to a commonword line are programmed simultaneously. A page can store one or moresectors. A sector may include user data and overhead data (also calledsystem data). Overhead data typically includes header information andError Correction Codes (ECC) that have been calculated from the userdata of the sector. The controller (or other component) calculates theECC when data is being programmed into the array, and also checks itwhen data is being read from the array. Alternatively, the ECC and/orother overhead data may be stored in different pages, or even differentblocks, than the user data to which they pertain. A sector of user datais typically 512 bytes, corresponding to the size of a sector inmagnetic disk drives. A large number of pages form a block, anywherefrom 8 pages, for example, up to 32, 64, 128 or more pages. Differentsized blocks, pages, and sectors can also be used.

FIG. 6 depicts one embodiment of a sense block 500, such as sense block500 in FIG. 5. An individual sense block 500 may be partitioned into acore portion, referred to as a sense module 580, and a common portion590. In one embodiment, there is a separate sense module 580 for eachbit line and one common portion 590 for a set of multiple sense modules580. In one example, a sense block will include one common portion 590and eight sense modules 580. Each of the sense modules in a group willcommunicate with the associated common portion via a data bus 572.

Sense module 580 comprises sense circuitry 570 that determines whether aconduction current in a connected bit line is above or below apredetermined threshold level. Sense module 580 also includes a bit linelatch 582 that is used to set a voltage condition on the connected bitline. For example, a predetermined state latched in bit line latch 582may result in the connected bit line being pulled to a state designatingprogram inhibit voltage (e.g., 1.5-3 V).

Common portion 590 comprises a processor 592, a set of data latches 594,and an I/O Interface 596 coupled between the set of data latches 594 anddata bus 520. Processor 592 performs computations. For example,processor 592 may determine the data stored in the sensed storageelement and store the determined data in the set of data latches. Theset of data latches 594 may be used to store data bits determined byprocessor 592 during a read operation or to store data bits importedfrom the data bus 520 during a program operation. The imported data bitsrepresent write data meant to be programmed into a memory array, such asmemory array 501 in FIG. 5. I/O interface 596 provides an interfacebetween data latches 594 and the data bus 520.

During a read operation or other storage element sensing operation, astate machine, such as state machine 512 in FIG. 5, controls the supplyof different control gate voltages to the addressed storage elements. Asit steps through the various predefined control gate voltagescorresponding to the various memory states supported by the memory, thesense module 580 may trip at one of these voltages and an output will beprovided from sense module 580 to processor 592 via bus 572. At thatpoint, processor 592 determines the resultant memory state byconsideration of the tripping event(s) of the sense module and theinformation about the applied control gate voltage from the statemachine via input lines 593. It then computes a binary encoding for thememory state and stores the resultant data bits into data latches 594.In another embodiment of the core portion, bit line latch 582 servesboth as a latch for latching the output of the sense module 580 and as abit line latch as described above.

During a programming operation, the data to be programmed is stored inthe set of data latches 594. The programming operation, under thecontrol of the state machine 512, comprises a series of programmingvoltage pulses applied to the control gates of the addressed storageelements. Each program pulse is followed by a read back (or verifyprocess) to determine if the storage element has been programmed to thedesired memory state. Processor 592 monitors the read back memory staterelative to the desired memory state. When the two are in agreement, theprocessor 592 sets the bit line latch 582 so as to cause the bit line tobe pulled to a state designating program inhibit voltage. This inhibitsthe storage element coupled to the bit line from further programmingeven if program pulses appear on its control gate. In other embodiments,the processor initially loads the bit line latch 582 and the sensecircuitry sets it to an inhibit value during the verify process.

Data latch stack 594 contains a stack of data latches corresponding tothe sense module. In one embodiment, there are three data latches persense module 580. The data latches can be implemented as a shiftregister so that the parallel data stored therein is converted to serialdata for data bus 520, and vice-versa. All the data latchescorresponding to a read/write block can be linked together to form ablock shift register so that a block of data can be input or output byserial transfer. In particular, the bank of read/write modules may beconfigured such that each of its set of data latches will shift data into or out of the data bus in sequence as if they are part of a shiftregister for the entire read/write block.

FIG. 7A depicts one embodiment of a set of threshold voltagedistributions for a four-state memory device in which each storageelement stores two bits of data. A first threshold voltage (Vth)distribution 700 is provided for erased (E-state) storage elements.Three Vth distributions 702, 704 and 706 represent programmed states A,B and C, respectively. In one embodiment, the threshold voltages in theE-state and the threshold voltages in the A, B and C distributions arepositive. In another embodiment, the threshold voltage distribution forthe E-state is negative, while the threshold voltage distributions forthe A-state, B-state and C-state distributions are positive.

Three read reference voltages, Vra, Vrb and Vrc, are also provided forreading data from storage elements. By testing whether the thresholdvoltage of a given storage element is above or below Vra, Vrb and Vrc,the system can determine the state, e.g., programming condition, thestorage element is in.

Further, three verify reference voltages, Vva, Vvb and Vvc, areprovided. When programming storage elements to the A-state, B-state orC-state, the system will test whether those storage elements have athreshold voltage greater than or equal to Vva, Vvb or Vvc,respectively.

In one embodiment, known as full sequence programming, storage elementscan be programmed from the E-state directly to any of the programmedstates A, B or C. For example, a population of storage elements to beprogrammed may first be erased so that all storage elements in thepopulation are in the E-state. A series of program pulses, such asdepicted in FIG. 7F, may then be used to program storage elementsdirectly into states A, B or C. While some storage elements are beingprogrammed from the E-state to the A-state, other storage elements arebeing programmed from the E-state to the B-state and/or from the E-stateto the C-state.

Another option is to use low and high verify levels for one or more datastates. For example, VvaL and Vva are lower and higher verify levels,respectively, for the A-state, VvbL and Vvb are lower and higher verifylevels, respectively, for the B-state, and VvcL and Vvc are lower andhigher verify levels, respectively, for the C-state. In some cases, VvcLis not used since reduced programming precision may be acceptable forthe highest state. During programming, when the Vth of a storage elementwhich is being programmed to the A-state as a target state exceeds VvaL,the programming speed of the storage element is slowed down, in a slowprogramming mode, such as by raising the associated bit line voltage toa level, e.g., 0.6-0.8 V, which is between a nominal program ornon-inhibit level, e.g., 0 V and a full inhibit level, e.g., 4-6 V. Thisprovides greater accuracy by avoiding large step increases in thresholdvoltage. When the Vth reaches Vva, the storage element is locked outfrom further programming. Similarly, when the Vth of a storage elementwhich is being programmed to the B-state as a target state exceeds VvbL,the programming speed of the storage element is slowed down, and whenthe Vth reaches Vvb, the storage element is locked out from furtherprogramming. Optionally, when the Vth of a storage element which isbeing programmed to the C-state as a target state exceeds VvcL, theprogramming speed of the storage element is slowed down, and when theVth reaches Vvc, the storage element is locked out from furtherprogramming. This programming technique has been referred to as a quickpass write or dual verify technique. Note that, in one approach, dualverify levels are not used for the highest state since some overshoot istypically acceptable for that state. Instead, the dual verify levels canbe used for the programmed states, above the erased state, and below thehighest state.

FIG. 7B depicts one embodiment of a first pass of a two-pass programmingtechnique. In this example, a multi-state storage element stores datafor two different pages: a lower page and an upper page. Four states aredepicted by repeating the threshold voltage distributions 700, 702, 704and 706 from FIG. 7A. These states, and the bits they represent, are:E-state (11), A-state (01), B-state (00) and C-state (10). For E-state,both pages store a “1.” For A-state, the lower page stores a “1” and theupper page stores a “0.” For B-state, both pages store “0.” For C-state,the lower page stores “0” and the upper page stores “1.” Note thatalthough specific bit patterns have been assigned to each of the states,different bit patterns may also be assigned.

In the first programming pass, the lower page is programmed for aselected word line WLn. If the lower page is to remain data 1, then thestorage element state remains at state E (distribution 700). If the datais to be programmed to 0, then the threshold voltage of the storageelements on WLn are raised such that the storage element is programmedto an intermediate (LM or lower-middle) state (distribution 705). In oneembodiment, after a storage element is programmed from the E-state tothe LM-state, its neighbor storage element on an adjacent word lineWLn+1 in the NAND string will then be programmed with respect to itslower page in a respective first programming pass of the adjacent wordline.

FIG. 7C depicts one embodiment of a second pass of the two-passprogramming technique referred to in FIG. 7B. The A-state storageelements are programmed from the E-state distribution 700 to the A-statedistribution 702, the B-state storage elements are programmed from theLM-state distribution 705 to the B-state distribution 704, and theC-state storage elements are programmed from the LM-state distribution705 to the C-state distribution 706.

FIG. 7D depicts one embodiment of a first pass of another two-passprogramming technique. In this example, referred to as foggy-fine (orcourse-fine) programming, the A-state, B-state and C-state storageelements are programmed from the E-state to distributions 712, 714 and716, respectively, using lower verify levels VvaL, VvbL and VvcL,respectively. This is the foggy (or course) programming pass. Arelatively large program voltage step size may be used, for instance, toquickly program the storage elements to the respective lower verifylevels.

FIG. 7E depicts one embodiment of a second pass of the two-passprogramming technique referred to in FIG. 7D. The A-state, B-state andC-state storage elements are programmed from the respective lowerdistributions to respective final distributions 702, 704 and 706,respectively, using the nominal, higher verify levels Vva, Vvb and Vvc,respectively. This is the fine programming pass. A relatively smallprogram voltage step size may be used, for instance, to slowly programthe storage elements to the respective final verify levels whileavoiding a large overshoot.

Although the programming examples depict four data states and two pagesof data, the concepts described herein may be applied to otherimplementations with more or fewer than four states and more or fewerthan two pages. For example, memory devices may utilize eight or sixteenstates per storage element. Moreover, in the example programmingtechniques discussed herein, the Vth of a storage element may be raisedgradually as it is programmed to a target data state. However,programming techniques may be used in which the Vth of a storage elementmay be lowered gradually as it is programmed to a target data state.

FIG. 7F depicts one embodiment of a series of program and verify pulseswhich are applied to a selected word line during a programmingoperation. A programming operation may include multiple program-verifyiterations, where each iteration applies one or more programming pulsesfollowed by one or more verify pulses (e.g., to verify or determine theprogramming state or the programming level of a memory cell) to aselected word line. In one embodiment, the programming pulses arestepped up in successive iterations. Moreover, each programming pulsemay include a first portion which has a pass voltage (Vpass) level,e.g., 6-8 V, followed by a second, highest amplitude portion at aprogramming voltage (Vpgm) level, e.g., 12-25 V. For example, asdepicted in FIG. 7F, a first, second, third, and fourth programmingpulses 800, 802, 804 and 806 have programming voltage levels of Vpgm1,Vpgm2, Vpgm3 and Vpgm4, respectively. One or more verify voltages 808,such as verify voltages Vva, Vvb and Vvc, may be provided after eachprogramming pulse. In some cases, one or more initial programming pulsesare not followed by verify pulses because it is not expected that anystorage elements could have reached the lowest program state (e.g.,A-state). Subsequently, in some cases, programming iterations may useverify pulses for the A-state, followed by programming iterations whichuse verify pulses for the A-states and B-states, followed by programmingiterations which use verify pulses for the B-states and C-states.

FIG. 8A depicts one embodiment of a vertical NAND string that connectsto a p-type doped source line. As depicted, the vertical NAND stringincludes a drain-side select gate transistor controlled by thedrain-side select gate line SGD, a drain-side dummy transistorcontrolled by the dummy word line WLDD, a plurality of memory celltransistors corresponding with word lines WL(n+1) through WL(n−1), asource-side dummy transistor controlled by the dummy word line WLDS, afirst source-side select gate transistor controlled by the source-sideselect gate line SGS, and a second source-side select gate transistorcontrolled by the source-side select gate line SGSB. The secondsource-side select gate transistor may correspond with the source-sideselect gate transistor depicted in FIG. 3A that directly connects to thesource line. The drain-side select gate transistor connects to a bitline 802 and the source-side select gate transistor that is controlledby the source-side select gate line SGSB connects to a source line 806.The source line 806 may comprise boron-doped silicon or boron-dopedpoly-silicon. The vertical NAND string includes a poly-silicon channelthat extends between the bit line 802 and the source line 806. Thevertical NAND string may include a vertical charge trapping layer, avertical MANOS layer, or a vertical TANOS layer arranged between theword lines and the poly-silicon channel. During a read operation, thethreshold voltage of a selected memory cell transistor 804 correspondingwith word line WL(n) may be determined by setting the unselected memorycell transistors within the vertical NAND string into conducting states(e.g., by applying Vread or 8V to the unselected word lines connected tothe unselected memory cell transistors) and applying a read bias voltage(e.g., by applying Vcgrv or 3V) to the selected word line WL(n)connected to the selected memory cell transistor. The read bias voltagemay be less than the voltages applied to the unselected word linesduring the read operation.

FIG. 8B depicts one embodiment of the vertical NAND string depicted inFIG. 8A during a read operation. As depicted, the bit line 802 has beenbiased to a bit line voltage of 1.0V and the source line 806 has beenbiased to a source line voltage of 2.2V. As the source line voltage isgreater than the bit line voltage, the read current flows from thesource line 806 to the bit line 802. In this case, the threshold voltageof the selected memory cell transistor 804 may be less than the voltageapplied to the selected word line WL(n) such that the read current flowstowards the bit line 802 to charge an integration capacitor or nodewithin a sense amplifier electrically connected to the bit line 802. Theaccumulation of charge due to the read current may be sensed by thesense amplifier in order to determine a data state corresponding withthe threshold voltage of the selected memory cell transistor 804. Thesensing circuitry may correspond with read/write circuits 565 in FIG. 5.

In one embodiment, if a data error is detected during the readoperation, then a controller, such as control circuitry 510 in FIG. 5 orcontroller 550 in FIG. 5, may adjust the source line voltage and/or thebit line voltage applied to the vertical NAND string during a subsequentread operation. The data error may be detected as an ECC error or as oneor more bit errors within data read during the read operation. In oneexample, if one or more bit errors are detected within the data readduring the read operation, then the controller may reduce the voltageapplied to the bit line 802 (e.g., reduce the bit line voltage from 1.0Vto 0.8V) and/or increase the source line voltage (e.g., increase thesource line voltage from 2.2V to 2.4V) and perform a subsequent readoperation in order to reduce the number of one or more bit errors. Inanother example, if the number of ECC errors or the number of bit errorswithin data read during the read operation is greater than a thresholdnumber of bit errors (e.g., is greater than two bit errors), then thevoltage difference between the source line voltage and the bit linevoltage may be increased during a subsequent read operation (e.g., thebit line voltage may remain at 1.0V while the source line voltage isincreased from 2.2V to 2.5V for the subsequent read operation).

In some embodiments, the memory cell transistors of the vertical NANDstring may comprise a vertical string of charge trap transistors. Inother embodiments, the memory cell transistors of the vertical NANDstring may comprise a vertical string of floating gate transistors. Inother embodiments, the memory cell transistors of the vertical NANDstring may comprise a vertical string of inverted charge traptransistors. In other embodiments, the memory cell transistors of thevertical NAND string may comprise a vertical string of inverted floatinggate transistors.

During a programming operation, electrons may be selectively injectedfrom the bit line 802 connected to the vertical NAND string depending onthe bit line voltage applied to the bit line during the programmingoperation. The programming of memory cell transistors may begin withmemory cell transistors arranged closest to the source line 806, such asthe memory cell transistor controlled by word line WL(n−1), and progresstowards the memory cell transistors arranged closest to the bit line802, such as the memory cell transistor controlled by word line WL(n+1).In one example, the memory cell transistor corresponding with word lineWL1 may be programmed first followed by the memory cell transistorcorresponding with word line WL2.

FIG. 8C depicts one embodiment of a conventional NAND string with ann-type source line 812 and an n-type bit line 814 during a readoperation. As depicted, the source line 812 has been set to a sourceline voltage of 0V and the bit line 814 has been set to a bit linevoltage of 0.5V. The unselected word lines corresponding with word linesWL(n−1) and WL(n+1) have been set to 7V, the drain-side select gate lineSGD has been set to 7V, the drain-side dummy word line WLDD has been setto 7V, the source-side dummy word line WLDS has been set to 7V, thesource-side select gate line SGS has been set to 7V, and the selectedword line WL(n) that controls the control gate of the selected memorycell transistor has been set to a read bias voltage Vr (e.g., 3V or3.5V). In this case, as the threshold voltage of the selected memorycell transistor is less than the read bias voltage Vr, a read current816 flows from the bit line 814 to the source line 812.

FIG. 8D depicts one embodiment of a NAND string with a p-type sourceline 822 and an n-type bit line 814 during a read operation. Asdepicted, the source line 822 has been set to a source line voltage of2.2V and the bit line 814 has been set to a bit line voltage of 1.0V.The unselected word lines corresponding with word lines WL(n−1) andWL(n+1) have been set to 7V, the drain-side select gate line SGD hasbeen set to 7V, the drain-side dummy word line WLDD has been set to 7V,the source-side dummy word line WLDS has been set to 9V, the source-sideselect gate line SGS has been set to 9V, and the selected word lineWL(n) that controls the control gate of the selected memory celltransistor has been set to a read bias voltage Vr (e.g., 3V). As thethreshold voltage of the selected memory cell transistor is less thanthe read bias voltage Vr, a read current 826 flows from the source line822 to the bit line 814.

FIG. 8E depicts one embodiment of the NAND string of FIG. 8D during asecond read operation in which the bit line voltage has been reducedfrom 1.0V to 0.5V. As the threshold voltage of the selected memory celltransistor is less than the read bias voltage Vr, a read current 828flows from the source line 822 to the bit line 814. The read current 828may be greater than the read current 826 in FIG. 8D. The adjustment madeto the bit line voltage 830 may be determined by a controller based on anumber of bit errors detected during a prior read operation. The numberof bit error may be determined by ECC circuitry. For example, if thenumber of bit errors during the prior read operation is greater than twobit errors, then the bit line voltage 830 may be reduced by 500 mV. Theadjustment made to the bit line voltage 830 may also be determined by acontroller based on a temperature during a read operation. For example,if a chip temperature is greater than a threshold temperature (e.g., isgreater than 70 degrees Celsius), then the bit line voltage 830 may bereduced by 200 mV.

FIG. 8F depicts one embodiment of an unselected NAND string within anunselected memory block. As the source line voltage applied to aselected memory block may also be applied to unselected memory blocks,the source line voltage applied to the source line 822 may cause leakagecurrent to flow within the unselected NAND string even though the wordlines and source-side select gate lines of the unselected NAND stringhave been set to 0V. To reduce the leakage current, the thresholdvoltages of the source-side select gate transistors of the selected andunselected memory blocks may be set to a negative threshold voltageprior to performance of read operations. As depicted, the thresholdvoltage of the source-side select gate transistor 832 has been set to−2V and the threshold voltage of the source-side dummy transistor 834has been set to 3V.

In some embodiments, the threshold voltage of the source-side selectgate transistor may be set based on the source line voltage applied tothe source line 822 during read operations. In one example, a controllermay determine that the source line voltage applied to source linesduring a first read operation will be a first voltage (e.g., 2.0V) andin response the controller may cause the threshold voltages of thesource-side select gate transistors to be set to a negative thresholdvoltage that has an absolute value that is greater than the firstvoltage (e.g., −2.2V). Prior to performance of a second read operation,the controller may determine that the source line voltage applied tosource lines during the second read operation will be a second voltagegreater than the first voltage (e.g., 2.5V) and in response thecontroller may cause the threshold voltages of the source-side selectgate transistors to be set to a negative threshold voltage that has anabsolute value that is greater than the second voltage (e.g., −3V).

FIG. 8G depicts one embodiment of an unselected NAND string within anunselected memory block. As the source line voltage applied to aselected memory block may also be applied to unselected memory blocks,the source line voltage applied to the source line 822 may cause leakagecurrent to flow within the unselected NAND string even though the wordlines and source-side select gate lines of the unselected NAND stringhave been grounded or set to 0V. To reduce the leakage current, thethreshold voltages of the source-side select gate transistors and thesource-side dummy transistors may be set to negative threshold voltagesprior to performance of read operations. As depicted, the thresholdvoltage of the source-side select gate transistor 832 has been set to−2V and the threshold voltage of the source-side dummy transistor 836has been set to −3V. In some cases, the threshold voltage applied to thesource-side dummy transistor that is adjacent to the source-side selectgate transistor of a NAND string may be less or more negative than thethreshold voltage applied to the source-side select gate transistor.

FIG. 8H depicts one embodiment of an unselected NAND string within anunselected memory block. As the source line voltage applied to aselected memory block may also be applied to unselected memory blocks toimprove physical layout efficiency, the source line voltage applied tothe source line 822 may cause leakage current to flow within theunselected NAND string even though the word lines and source-side selectgate lines of the unselected NAND string have been grounded. To reducethe leakage current, the threshold voltages of the source-side dummytransistors of the selected and unselected memory blocks may be set to anegative threshold voltage prior to performance of read operations. Asdepicted, the threshold voltage of the source-side select gatetransistor 838 has been set to 2V and the threshold voltage of thesource-side dummy transistor 836 has been set to −3V. In some cases, thethreshold voltage of the source-side dummy transistor 836 may be set toa negative threshold voltage that has an absolute value that is greaterthan the source line voltage applied to the source line 822. Forexample, if the source line voltage is 2.2V, then the threshold voltageof the source-side dummy transistor 836 may be set to −2.3V or −3V.

FIGS. 9A-9B depict a flowchart describing one embodiment of a processfor performing read operations. In one embodiment, the process of FIGS.9A-9B may be performed by a non-volatile storage system, such as thenon-volatile storage system 596 in FIG. 5. In another embodiment, theprocess of FIGS. 9A-9B may be performed by control circuitry, such asthe control circuitry 510 in FIG. 5.

In step 902, a source line voltage to be applied to a source lineconnected to a NAND string during a read operation is determined. In oneexample, a controller may access a lookup table stored in non-volatilememory in order to identify the source line voltage to be applied. Thesource line voltage to be applied during the read operation may dependon a chip temperature. For example, the source line voltage may comprisea first voltage (e.g., 2.2V) if the chip temperature is below athreshold temperature (e.g., is below 45 degrees Celsius) or a secondvoltage greater than the first voltage if the chip temperature is abovethe threshold temperature. The controller may cause the source linevoltage to be applied to the source line, such as source line 822 inFIG. 8D. In step 904, a threshold voltage of a source-side select gatetransistor of the NAND string is programmed, erased, or set based on thesource line voltage. The threshold voltage of a programmable transistormay be set to a desired threshold voltage via an erase operation or aprogramming operation. In one embodiment, the controller may set thethreshold voltage of the source-side select gate transistor to anegative threshold voltage that has an absolute value that is greaterthan the source line voltage to be applied to the source line during theread operation.

In step 906, a bit line voltage to be applied to a bit line connected tothe NAND string during the read operation is determined. The bit linevoltage may be less than the source line voltage. In one example, thebit line voltage may comprise 1.0V and the source line voltage maycomprise 2.2V. In step 908, an unselected word line voltage to beapplied to an unselected word line connected to a control gate of asecond memory cell transistor of the NAND string during the readoperation is determined. In step 910, a selected word line voltage to beapplied to a selected word line connected to a control gate of a firstmemory cell transistor of the NAND string during the read operation isdetermined. Referring to FIG. 8D, the selected word line may correspondwith the word line WL(n) and the unselected word line may correspondwith the word line WL(n+1). In step 912, the source line is set to thesource line voltage. In step 914, the bit line is set to the bit linevoltage. The bit line voltage may be less than the source line voltage.In some cases, the voltage difference between the source line voltageand the bit line voltage may be increased in order to improve sensingmargin or to improve the ability for sense amplifiers to determinestored data states of memory cell transistors. In step 916, theunselected word line is set to the unselected word line voltage. In step918, the selected word line is set to the selected word line voltage.

In step 920, the read operation is performed while the source line isset to the source line voltage, the bit line is set to the bit linevoltage, the unselected word line is set to the unselected word linevoltage, and the selected word line is set to the selected word linevoltage. The read operation may involve a sense amplifier or read/writecircuits, such as read/write circuits 565 in FIG. 5, that are configuredto determine a threshold voltage level and a corresponding data statefor a selected memory cell transistor. In step 922, a read error isdetected during the read operation. The read error may be detected byerror detection and correction circuitry and may be detected if one ormore bit errors are detected within the data read from NAND stringsduring the read operation. In step 924, the bit line voltage is updatedor adjusted in response to detection of the read error. In one example,a controller may reduce the bit line voltage applied to bit lines duringa second read operation that is performed subsequent to the readoperation in response to detecting that one or more bit errors haveoccurred during the read operation. In step 926, a second read operationis performed while the source line is set to the source line voltage,the bit line is set to the updated bit line voltage, the unselected wordline is set to the unselected word line voltage, and the selected wordline is set to the selected word line voltage.

FIG. 9C depicts a flowchart describing another embodiment of a processfor performing a read operation. In one embodiment, the process of FIG.9C may be performed by a non-volatile storage system, such as thenon-volatile storage system 596 in FIG. 5. In another embodiment, theprocess of FIG. 9C may be performed by control circuitry, such as thecontrol circuitry 510 in FIG. 5.

In step 932, a source line voltage to be applied to a source lineconnected to a NAND string during a read operation is determined. Thesource line may correspond with a p-type material, such as boron-dopedpolysilicon. In step 934, a bit line voltage to be applied to a bit lineconnected to the NAND string during the read operation is determined.The bit line may correspond with an n-type material, such asphosphorus-doped polysilicon. In step 936, a threshold voltage of asource-side select gate transistor of the NAND string is set (e.g., viaan erase operation) based on the source line voltage to be applied tothe source line during the read operation. In step 938, a thresholdvoltage of a source-side dummy transistor of the NAND string isprogrammed, erased, or set based on the source line voltage to beapplied to the source line during the read operation. In step 940, theread operation is performed while the source line is biased to thesource line voltage and the bit line is biased to the bit line voltagesubsequent to setting the threshold voltages of the source-side selectgate transistor and the source-side dummy transistor. Both the thresholdvoltages of the source-side select gate transistor and the source-sidedummy transistor may comprise negative threshold voltages in order toreduce leakage currents in NAND strings of unselected memory blocks.

In step 942, it is detected that a leakage current through unselectedNAND strings is greater than a threshold current (e.g., is greater than0.1 mA). The leakage current may be detected using on-chip leakagedetection circuitry. In step 944, the threshold voltage of thesource-side select gate transistor of the NAND string is reduced or mademore negative in response to detection that the leakage current isgreater than the threshold current. In one embodiment, a controller maydetect that the leakage current has exceeded 0.1 mA and in response maycause the threshold voltage of the source-side select gate transistorsto be set to threshold voltages that are 0.5V more negative. Forexample, the controller may cause the threshold voltage of a source-sideselect gate transistor to be adjusted from −2V to −2.5V.

In other embodiments, the controller may detect that the leakage currenthas exceeded the threshold current and in response may reduce thethreshold voltages of source-side select gate transistors and reduce thesource line voltages applied to source lines during one or moresubsequent read operations. In one example, the controller may cause thethreshold voltages of source-side select gate transistors to be mademore negative by 500 mV and the source line voltages applied to sourcelines during the subsequent read operations to be reduced by 300 mV.

FIG. 9D depicts a flowchart describing an alternative embodiment of aprocess for performing a read operation. In one embodiment, the processof FIG. 9D may be performed by a non-volatile storage system, such asthe non-volatile storage system 596 in FIG. 5. In another embodiment,the process of FIG. 9D may be performed by control circuitry, such asthe control circuitry 510 in FIG. 5.

In step 952, a source line voltage to be applied to a source lineconnected to a NAND string during a read operation is determined. Instep 954, a bit line voltage to be applied to a bit line connected tothe NAND string during the read operation is determined. The source linevoltage to be applied to the source line and the bit line voltage to beapplied to the bit line may be determined via a lookup table stored innon-volatile memory. In step 956, a threshold voltage level for asource-side select gate transistor of the NAND string is determinedbased on the source line voltage and the bit line voltage to be appliedduring the read operation.

In one embodiment, the threshold voltage level for the source-sideselect gate transistor may depend on a voltage difference between thesource line voltage and the bit line voltage. For example, if thevoltage difference between the source line voltage and the bit linevoltage is greater than a first voltage (e.g., is greater than 2V), thenthe threshold voltage level for the source-side select gate transistormay be set to a first negative threshold voltage (e.g., to −2V);however, if the voltage difference between the source line voltage andthe bit line voltage is not greater than the first voltage (e.g., isless than 2V), then the threshold voltage level for the source-sideselect gate transistor may be set to a second negative threshold voltage(e.g., to −1.5V) that is less negative than the first negative thresholdvoltage.

In another embodiment, the threshold voltage level for the source-sideselect gate transistor may be set to a negative threshold voltage thathas an absolute value that is greater than the source line voltage. Forexample, the threshold voltage level for the source-side select gatetransistor may be set to negative 2.5V or negative 3.0V if the sourceline voltage is positive 2V.

In step 958, a threshold voltage of a source-side select gate transistoris programmed, erased, or set to the threshold voltage level. In step960, it is detected that a chip temperature has exceeded a thresholdtemperature. An on-chip temperature sensor may be used to detect thatthe chip temperature has exceeded the threshold temperature (e.g., isgreater than 55 degrees Celsius). In step 962, the threshold voltage ofthe source-side select gate transistor is reduced (e.g., is reduced by500 mV) in response to detection that the chip temperature has exceededa threshold temperature. As leakage currents may increase with anincrease in chip temperature, the reduction in the threshold voltage ofthe source-side select gate transistor may reduce the leakage currents.In step 964, the read operation on the NAND string is performed whilethe source line is biased to the source line voltage and the bit line isbiased to the bit line voltage subsequent to reducing the thresholdvoltage of the source-side select gate transistor of the NAND string.

One embodiment of the disclosed technology includes a NAND stringincluding a drain-side select gate transistor connected to a bit lineand a source-side select gate transistor connected to a source line, andone or more control circuits in communication with the bit line and thesource line. The one or more control circuits configured to determine asource line voltage to be applied to the source line during a readoperation and determine a threshold voltage level for the source-sideselect gate transistor based on the source line voltage. The one or morecontrol circuits configured to set a threshold voltage of thesource-side select gate transistor to the threshold voltage level priorto the read operation and set the bit line to a bit line voltage lessthan the source line voltage applied to the source line during the readoperation.

One embodiment of the disclosed technology includes acquiring a sourceline voltage to be applied to a source line connected to a NAND stringduring a read operation and acquiring a bit line voltage to be appliedto a bit line connected to the NAND string during the read operation.The bit line voltage is less than the source line voltage. The methodfurther comprises determining a threshold voltage level for asource-side select gate transistor of the NAND string based on thesource line voltage to be applied to the source line during the readoperation, setting a threshold voltage of the source-side select gatetransistor to the threshold voltage level prior to performing the readoperation, and performing the read operation to determine a data stateof a selected memory cell transistor within the NAND string while thesource line is set to the source line voltage and the bit line is set tothe bit line voltage subsequent to setting the threshold voltage of thesource-side select gate transistor to the threshold voltage level basedon the source line voltage applied to the source line during the readoperation.

One embodiment of the disclosed technology includes a NAND string andone or more control circuits. The NAND string including a first memorycell transistor and a source-side select gate transistor with a negativethreshold voltage that is connected to a p-type doped source line. Theone or more control circuits in communication with the NAND string. Theone or more control circuits configured to determine a selected wordline voltage to be applied to a selected word line connected to acontrol gate of the first memory cell transistor and determine anunselected word line voltage greater than the selected word line voltageto be applied to an unselected word line connected to a control gate ofa second memory cell transistor of the NAND string. The one or morecontrol circuits configured to read the first memory cell transistorwhile the selected word line is set to the selected word line voltageand the unselected word line is set to the unselected word line voltage.

One embodiment of the disclosed technology includes acquiring a selectedword line voltage to be applied to a selected word line connected to acontrol gate of a first memory cell transistor of a NAND string. TheNAND string includes a source-side select gate transistor with anegative threshold voltage that is connected to a p-type doped sourceline. The method further comprises acquiring an unselected word linevoltage to be applied to an unselected word line connected to a controlgate of a second memory cell transistor of the NAND string. Theunselected word line voltage is greater than the selected word linevoltage. The method further comprises determining a data state of thefirst memory cell transistor while the selected word line is set to theselected word line voltage and the unselected word line is set to theunselected word line voltage.

For purposes of this document, it should be noted that the dimensions ofthe various features depicted in the figures may not necessarily bedrawn to scale.

For purposes of this document, reference in the specification to “anembodiment,” “one embodiment,” “some embodiments,” or “anotherembodiment” may be used to describe different embodiments and do notnecessarily refer to the same embodiment.

For purposes of this document, a connection may be a direct connectionor an indirect connection (e.g., via another part). In some cases, whenan element is referred to as being connected or coupled to anotherelement, the element may be directly connected to the other element orindirectly connected to the other element via intervening elements. Whenan element is referred to as being directly connected to anotherelement, then there are no intervening elements between the element andthe other element.

For purposes of this document, the term “based on” may be read as “basedat least in part on.”

For purposes of this document, without additional context, use ofnumerical terms such as a “first” object, a “second” object, and a“third” object may not imply an ordering of objects, but may instead beused for identification purposes to identify different objects.

For purposes of this document, the term “set” of objects may refer to a“set” of one or more of the objects.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

What is claimed is:
 1. A non-volatile storage system, comprising: a NANDstring including a drain-side select gate transistor connected to a bitline and a source-side select gate transistor connected to a sourceline; and one or more control circuits in communication with the bitline and the source line, the one or more control circuits configured todetermine a source line voltage to be applied to the source line duringa read operation and determine a threshold voltage level for thesource-side select gate transistor based on the source line voltage, theone or more control circuits configured to set a threshold voltage ofthe source-side select gate transistor to the threshold voltage levelprior to the read operation and set the bit line to a bit line voltageless than the source line voltage applied to the source line during theread operation.
 2. The non-volatile storage system of claim 1, wherein:the bit line comprises an n-type doped bit line and the source linecomprise a p-type doped source line.
 3. The non-volatile storage systemof claim 1, wherein: the threshold voltage of the source-side selectgate transistor comprises a negative threshold voltage.
 4. Thenon-volatile storage system of claim 1, wherein: the threshold voltageof the source-side select gate transistor comprises a negative thresholdvoltage with an absolute value of the negative threshold voltage that isgreater than the source line voltage.
 5. The non-volatile storage systemof claim 1, further comprising: determining a threshold voltage of asource-side dummy transistor of the NAND string based on the source linevoltage, the source-side dummy transistor is arranged between thesource-side select gate transistor and a memory cell transistor of theNAND string; and setting the threshold voltage of the source-side dummytransistor prior to performance of the read operation.
 6. Thenon-volatile storage system of claim 5, wherein: the threshold voltageof the source-side dummy transistor comprises a negative thresholdvoltage that is less than the threshold voltage of the source-sideselect gate transistor.
 7. The non-volatile storage system of claim 1,wherein: the NAND string comprises a vertical NAND string.
 8. Thenon-volatile storage system of claim 7, wherein: the vertical NANDstring includes a plurality of charge trap transistors.
 9. Thenon-volatile storage system of claim 7, wherein: the vertical NANDstring includes a plurality of inverted charge trap transistors.
 10. Thenon-volatile storage system of claim 1, wherein: the one or more controlcircuits configured to determine the threshold voltage level for thesource-side select gate transistor based on the source line voltage andthe bit line voltage.
 11. A method for operating a non-volatile memory,comprising: acquiring a source line voltage to be applied to a sourceline connected to a NAND string during a read operation; acquiring a bitline voltage to be applied to a bit line connected to the NAND stringduring the read operation, the bit line voltage is less than the sourceline voltage; determining a threshold voltage level for a source-sideselect gate transistor of the NAND string based on the source linevoltage to be applied to the source line during the read operation;setting a threshold voltage of the source-side select gate transistor tothe threshold voltage level prior to performing the read operation; andperforming the read operation to determine a data state of a selectedmemory cell transistor within the NAND string while the source line isset to the source line voltage and the bit line is set to the bit linevoltage subsequent to setting the threshold voltage of the source-sideselect gate transistor to the threshold voltage level.
 12. The method ofclaim 11, wherein: the source line corresponds with a p-type material;and the bit line corresponds with an n-type material.
 13. The method ofclaim 11, wherein: the threshold voltage of the source-side select gatetransistor comprises a negative threshold voltage.
 14. The method ofclaim 11, wherein: the threshold voltage of the source-side select gatetransistor comprises a negative threshold voltage with an absolute valueof the negative threshold voltage that is greater than the source linevoltage.
 15. The method of claim 11, further comprising: determining athreshold voltage of a source-side dummy transistor of the NAND stringbased on the source line voltage, the source-side dummy transistor isarranged between the source-side select gate transistor and the selectedmemory cell transistor of the NAND string; and setting the thresholdvoltage of the source-side dummy transistor to a negative thresholdvoltage prior to performing the read operation.
 16. The method of claim11, wherein: the NAND string comprises a vertical NAND string.
 17. Themethod of claim 16, wherein: the vertical NAND string includes aplurality of charge trap transistors.
 18. The method of claim 16,wherein: the vertical NAND string includes a plurality of invertedcharge trap transistors.
 19. An apparatus, comprising: a NAND stringincluding a drain-side select gate transistor connected to an n-typedoped bit line and a source-side select gate transistor connected to ap-type doped source line; and one or more control circuits incommunication with the bit line and the source line, the one or morecontrol circuits configured to acquire a source line voltage to beapplied to the source line during a read operation and determine athreshold voltage level for the source-side select gate transistor basedon a voltage difference between the source line voltage and the bit linevoltage, the one or more control circuits configured to set a thresholdvoltage of the source-side select gate transistor to the thresholdvoltage level prior to the read operation and set the bit line to a bitline voltage less than the source line voltage applied to the sourceline during the read operation.
 20. The apparatus of claim 19, wherein:the threshold voltage level comprises a negative threshold voltage withan absolute value of the negative threshold voltage that is greater thanthe source line voltage applied to the source line during the readoperation.